1. Field of the Invention
This invention relates to imagable and imaged members. In particular, but not exclusively, this invention relates to printing form, electronic part and mask precursors and imaged printing forms, electronic parts and masks.
2. Background Information
Imaged articles, such as printing forms, electronic parts and masks, conventionally comprise a substrate onto which has been coated a film forming radiation sensitive composition, the composition having been image-wise exposed to radiation of suitable wavelength, and developed to produce the imaged member.
A common form of printing plate used in the printing industry is the lithographic printing forms. Many lithographic printing plates are imaged within imagesetters. In the manufacture of such lithographic plates, rolls or sheets of flexible material are generally fed into the image setting apparatus and digitally imaged within the imagesetter before being forwarded to prepress processing and onto a printing press. Imagesetters generally include one or more rollers or angular components around which the flexible precursor must bend during imaging. Thus, the substrate of the precursor must be flexible enough to allow passage over rollers and angular components. As such, typical substrates used for lithographic printing forms include flexible polyester sheets and paper sheets. The inherent flexibility of these materials allows the precursor to travel round rollers and angular components with relatively little damage to the structure of the precursor and imaged precursor.
However, problems arise once the imaged precursor has traveled through the imagesetter and undergoes prepress processing and clamping to the press cylinders of the printing press. In order for efficient printing to be effected, the imaged member must be securely clamped to the printing press, and pulled taut such that there are no inconsistencies in the relief of the plate on the press. Generally, such plates are pulled taut by the practice of clamping both the leading and trailing edge of the plate to the print cylinder. The practice of clamping and tightening of the imaged member can easily stretch flexible substrates such as polyester and paper when mechanically stressed. Stretching of the substrate induces stretching of the imaged coating on the substrate, which distorts any image printed from that particular plate. Furthermore, there is a danger that, with particularly flexible substrates such as paper, that tightening of the imaged member on the printing press will lead to tearing of the substrate with a subsequent loss of image.
Thus, the inherent flexibility of such plates whilst advantageous for the process of imaging in a imagesetter, also confers inherent dimensionally instability on those substrates, which can be disadvantageous when mounting the substrate on a printing cylinder.
Other more dimensionally stable forms of substrate can be used, such as aluminum plates, but their inherent inflexibility considerably increases the difficulty of the aluminum printing forms being passed through imagesetters. As imagesetters are used by many printing operatives around the globe, the cost of converting from using film setting equipment to equipment which can utilize inflexible aluminum plates can be financially prohibitive.
Other imaged members such as flexographic printing plates and printed circuit boards are commonly made from thick sheets of flexible plastic substrate. The thickness of the sheet is used to effect sufficient dimensional stability to the substrate against stresses encountered during use. The need for thick substrates, is relatively expensive and there is a desire in the industry to reduce substrate thickness whilst maintaining dimensional stability.
For flexographic plates in particular, historically these imaged members have been imaged by using film as a masking medium. The need for separate masking medium is relatively labor intensive and enhances the cost of producing such flexographic plates. The flexographic printing plate industry has consequently been looking for ways to reduce costs and labor intensity of producing such plates. One method of reducing costs and labor, would be to adopt the digital imaging using readily available film setting equipment, which eliminates the need for masking medium and its associated costs. However, the thickness and relative inflexibility of the substrates used in flexographic printing, compared to the flexible substrates used in lithographic printing, prevents their use in conventional imagesetting equipment.
Many attempts have been made to improve the dimensional stability of flexible substrates which allow the substrate to pass through an imagesetter but which after imaging is dimensionally stable enough to endure the mechanical stress of being tightened over a printing cylinder or printing surface. In particular, many flexible substrates are laminated with a dimensionally stable support such as an aluminum surface or dimensionally stable plastic surface, which laminated support is generally of a very thin construction in order that the flexible support may pass through an imagesetter. Examples of laminated flexible supports include those disclosed in U.S. Pat. No. 4,092,925 (Fromson), U.S. Pat. No. 2,048,964 (Osbourne), EP 690349 A1 (Dupont), U.S. Pat. No. 4,032,684 (Dunnington et el), WO 93/10979 (Aloisi), U.S. Pat. No. 3,979,212 (Peters et el), EP 644064 A (Agfa), EP 807534 A (Agfa) and WO 98/53371 (Identity Group Inc.). In each of these documents, aluminum or plastic sheeting is laminated to an imaged or imagable member in order to increase its dimensional stability when mounted on a printing press. The cost of the lamination materials, laminating equipment and processing can be relatively expensive, and time consuming.
JP 3073392 discloses a lithographic printing plate comprising a paper base in which an electron beam hardenable resin is impregnated, and to which is coated an electron beam hardenable resin layer. The impregnated paper and resin layer are then irradiated using an electron beam in order to harden the resins, in order to increase the stability of the printing plates. Again, the cost of providing impregnated resin and a further electron beam hardenable resin coating is relatively expensive, and time consuming to perform.
There is therefore a need in the lithographic printing, flexographic printing and printed circuit board industries for a substrate which is imagesetter compatible in its flexibility, but which after imaging is dimensionally stable enough to endure the mechanical stress of being tightened over a printing cylinder or printing surface, in the case of circuit boards, and which does not involve expensive and time consuming treatment in order to render the substrate dimensionally stable.
It is therefore an aim of preferred embodiments of the present invention to overcome or mitigate at least one of the problems of the prior art, or other problems, whether expressly described hereinabove or not.
According to a first aspect of the present invention there is provided a precursor to an imaged member comprising a dimensionally stable substrate including an imagable coating on a surface thereof, wherein the uncoated dimensionally stable substrate comprises dimensionally stable paper comprising at least one of the following characteristics:
(i) an elastic yield such that the tensile force required to exceed the elastic yield is greater than 60 Nmmxe2x88x922;
(ii) a percentage elongation of the paper under a tensile load or strain at the elastic yield point less than 1%; and
(iii) a Young""s Modulus under tensile load greater than 7 GPa.
By xe2x80x9celastic yieldxe2x80x9d we mean the limit to which the substrate can be strained with a load and still return to its original length on unloading.
By dimensional stability we mean the structural capability of the substrate to resist damage from mechanical stress. Resistance may be against stretching, breaking, tearing, distortion, indentation, warping, buckling or contraction caused by mechanical stress, for example.
In the case of an anisotropic paper the values of (i), (ii) and (iii) are the minimum values of the paper in any one direction.
The dimensionally stable paper suitably comprises characteristics (i) and (ii); (ii) and (iii); or (i) and (iii), but preferably comprises all three characteristics (i), (ii) and (iii).